1. Field of the Invention
This invention pertains to the general field of reclamation of contaminated materials. In particular, it provides a new method and apparatus for the treatment of contaminated soil by breaking up and recovering the contaminant from the soil particles in a process employing a novel application of gas-sparged centrifugal field separation.
2. Description of the Prior Art
The ever increasing use of chemicals and chemical processing around the world to satisfy our civilization's appetite for comfort and goods produces a continuous output of contaminated materials that have to be reckoned with in an environmentally safe and economically feasible manner. Historically, contaminated materials have been treated as waste and stored permanently in landfills and similar dump sites. In recent years, though, we have come to recognize the environmental hazards associated with the unchecked use of the body of the earth as a permanent waste repository. We have discovered that underground water reservoirs can be contaminated by seepage and that the atmosphere can be polluted by evaporation. We have also been forced to accept the physical limitations involved with the disposal of huge amounts of waste in dedicated sites scattered around the planet. Finally, we have begun to appreciate the value of some of the materials discarded and the potential for recycling them into productive uses.
Accordingly, during the last couple of decades there has existed a tremendous impetus for developing commercial equipment for the reclamation of contaminated material. In particular, several technologies have been commercialized to reclaim soil contaminated by chemicals used in petroleum refining and other industrial activities. These chemicals include, for example, petroleum hydrocarbons such as diesel and other fuels, polycyclic aromatic hydrocarbons (PAH's), halogenated solvents, polychlorinated biphenols (PCB's), toxiphenes, and pesticides.
One often used remediation technology involves incineration of the contaminated soil in order to free it from the contaminant particles, which are burnt away. This process requires large amounts of energy to carry out the combustion and retains the problem of air quality control associated with the production of incineration exhausts and residues. As a result, the process is very expensive and not cost effective for most applications involving large amounts of contaminated soil.
An alternative remediation method is based on in situ biological treatment of the contaminated soil. The contaminant is exposed to bacteria and other agents that produce chemical changes and result in its ultimate elimination. Thus, relatively clean soil is left at the end of the process. This method is normally slow and requires long treatment periods to achieve satisfactory decontamination, resulting in prolonged environmental exposure and monitoring requirements. In addition, the method can be ineffective for soils containing high levels of toxic organic pollutants.
Recently developed technologies have utilized known unit operations from the mineral processing industry to separate and recover the soil from the contaminant. For example, a process developed by the German company Harbauer and marketed under the trademark "Harbauer PB3" is capable of cleaning contaminated soil and rubble to the point that the cleaned material can be refilled on site for normal use. The contaminant is washed out of the polluted material in an extraction unit that utilizes mechanical energy to break up the bonds between the contaminant and the soil particles, so that the contaminant can be entrained in a liquid extraction phase, generally water. This liquid extraction phase is then concentrated to recover reusable water through several steps involving oil separation, flotation, desorption, and filtration and adsorption over activated carbon. The resulting contaminant sludge, which also contains all soil particles up to 15 microns in diameter, is disposed of in landfills as nonrecoverable waste. The decontaminated soil is also concentrated through several dewatering steps involving traditional hydrocyclone separation and is delivered as a reusable output. Thus, this process reclaims polluted soil particles of sizes greater than 15 microns, but leaves smaller soil and contaminant particles as non-recyclable waste.
Another recent development in the area of soil remediation is the technology marketed under the trade mark "BSTS" by the U. S. company Biotrol. The process is centered around a countercurrent scrubbing system that uses water as the separation medium. The scrubbing process consists of a number of screening, attrition, classification, froth flotation, and dewatering steps. In addition, a biological water treatment process is used to degrade the contaminants in the wash water from the soil scrubbing unit to nondetectable levels. In test runs where this process has been applied to soil contaminated with wood preservative, up to 95 percent of the contaminant was removed.
It is apparent from the foregoing that all such processes involve multistage unit operations requiring relatively large, sophisticated and expensive treatment plants. Therefore, there is a need for a simpler decontamination process that can be carried out on portable equipment in an efficient and cost effective manner. The present invention addresses this need and utilizes a novel approach to gas-sparged centrifugal classification to achieve the desired results.
In particular, this invention discloses a new method and apparatus founded on the well established principle of cyclone phase separation. When a suspension of solid particles in a fluid is fed tangentially into the top of a conical chamber, it acquires a spinning trajectory and the tangential velocity of the particles tends to carry them toward the periphery of the chamber. The result is a downward spiral path of increasing radius until the particles reach the boundary. The particles then continue their spiraling descent down the wall while the fluid moves upward in the central core. Because at high tangential velocities the outward force on a particle is many times greater than the force of gravity, cyclones accomplish more rapid and effective separation than gravitational settling chambers.
When the particles in the feed to a cyclone vary in size, the heavier particles have greater kinetic energy for a given tangential velocity and, therefore, reach the boundary more quickly than the lighter particles. By appropriately designing its geometry and operating conditions, a cyclone can thus be used to separate particles by size. When the fluid in the feed is liquid, this separation can be further aided by the technique of sparging gas bubbles into the cyclone vortex from the interior of the cyclone wall. It has been found that these bubbles tend to create turbulence at the boundary layer with the liquid in the vortex, thereby increasing the probability of collisions with particles in suspension, the lighter fraction of which adheres to and is entrained by the bubbles to form a foam that migrates to the center of the cyclone because of its relatively lighter weight. Therefore, gas sparging enhances the classification properties of cyclones.
In U.S. Pat. No. 4,279,743 (1981), Miller describes a hydrocyclone incorporating an air sparging system to improve or control the size separation of particles present in the feed. This hydrocyclone can also be used to separate hydrophobic particles from hydrophilic particles in the centrifugal field. The feed slurry is pumped tangentially into the top portion of the apparatus according to traditional cyclone operation and air bubbles are introduced from the wall midway down the chamber.
U.S. Pat. No. 4,397,741 to Miller (1983) teaches the use of air injection in a centrifugal field to provide flotation to separate solid particles from a fluid. Instead of a traditional cyclone construction, the apparatus consists of a cylindrical vessel with a tangential inlet and a tangential outlet. The particulate suspension swirls around the inner surface of the vessel and air is sparged into it through a porous wall. Because of the high probability of collision between the air bubbles and the particles in suspension, relatively light bubble/particle aggregates are formed that migrate toward the center of the vessel where they are collected in the form of a froth.
In U.S. Pat. No. 4,399,027 (1983), Miller applies the air sparging technique to achieve satisfactory flotation in known hydrocyclone apparatus. Several configurations are described to embody the invention for various types of hydrocyclones.
In U.S. Pat. No. 4,744,890 (1988), Miller et al. illustrate another gas-sparged flotation apparatus to separate particles from mineral ore slurries. The device includes a cylindrical vessel having a tangential inlet at its upper end and an annular outlet at its lower end. The annular outlet permits the smooth exit of the fluid discharged from the flotation vessel in order to avoid disturbance of fluid flow within the vessel itself. The vessel includes an adjustable froth pedestal at its lower end to support the froth column formed by the gas sparged into the swirling slurry and to isolate the column from the fluid discharge.
Finally, U.S. Pat. No. 4,838,434 (1988) to Miller et al. shows further improvements in air sparged hydrocyclone flotation resulting from equipping the vessel with generally conical froth pedestals. These new configurations produce increased flotation yields, particularly in the recovery of fines from coal beneficiation processes.
Thus, while the prior art teaches us several principles of general application in the beneficiation of materials, it has not produced a technology for the practical reclamation of contaminated soil. Therefore, there exists a need for affordable soil remediation apparatus that can be used on site, is capable of a commercially viable throughputs, and is based on an environmentally acceptable process.